As the performance of semiconductor integrated circuits improves, copper is replacing aluminum and becoming the material of choice for interconnects due to its lower resistivity and better electro-migration resistance. However, unlike aluminum which forms a native protective oxide layer, copper is more susceptible to corrosion. The copper corrosion can occur during the copper interconnect manufacturing processes due to its exposure to the chemical or ambient environment, and it can be further enhanced by the exposure to the light (photovoltaic effect) due to its connection to p-n junctions on the wafer. The copper corrosion usually happens between copper CMP, where a flesh copper surface is exposed, and the next process step, such as the passivation layer or etch stop layer (SiN, SiC) deposition. In the prior art, one way to reduce copper corrosion is through the use of corrosion inhibitor, such as Benzotriazole (BTA), in the manufacturing process.
The difficulty in using BTA for corrosion prevention is in controlling the applied amount due to the fact that the BTA applied during or post CMP process on the wafer surface has to be removed in the vacuum deposition tool prior to the application of the passivation layer. If the applied amount of BTA is not enough or the uniformity of the applied BTA layer is not good then the desired effect of corrosion prevention cannot be achieved. On the other hand, if too much BTA is applied to the wafer surface then it is very difficult to remove it completely before the next process step. BTA residue on the wafer will often cause defectivity and impact device yield and reliability. Additionally, the removal of BTA in the vacuum deposition tool has resulted in high maintenance costs and long tool down times. Another way to reduce copper corrosion, particularly the corrosion due to photovoltaic effect is to darken the environment during the manufacturing process. However, this increases the manufacture cost, is difficult to implement, and it does not completely eliminate corrosion. Yet another method to reduce copper corrosion is to impose a time window between copper CMP and the next process step. The controlled time delay between the two processes is simply to reduce as much as possible the exposure time of the copper interconnect to ambient environment to minimize the corrosion. This increases the manufacture cost and is difficult to implement; and yet it does not completely elimination copper corrosion. Hence, there is a need to further improve corrosion resistance of copper in the integrated circuits manufacturing process. Furthermore, it is highly desirable to replace the BTA with a layer or layers that can be applied more uniformly, have higher corrosion resistance than the copper interconnect, can selectively coat the copper interconnects, and does not need to be removed at the next passivation layer deposition.